1. Field of the Invention
The present invention relates to an active poly-phase filter in which a difference output between output voltages of two terminals and a difference output between output voltages of other two terminals are produced in response to high-frequency power applied to the filter so as to have a phase difference of π/2 radians. This invention is useful for a circuit composing a part of an image rejection filter which removes a component of an image frequency in the frequency conversion between a high-frequency band (i.e., radio frequency band) and an intermediate frequency band.
2. Description of Related Art
It is known that two difference outputs, having phases different from each other by π/2 radians, are produced in a poly-phase filter having four input terminals and four output terminals. FIG. 1 is a circuit view of a poly-phase filter having four input terminals Vi−1, Vi−2, Vi−3 and Vi−4 and four output terminals Vo−1, Vo−2, Vo−3 and Vo−4.
As shown in FIG. 1, in a poly-phase filter 15, a capacitor C1 is located between the input terminal Vi−1 and the output terminal Vo−1, a resistor R1 is located between the input terminal Vi−2 and the output terminal Vo−1, a capacitor C2 is located between the input terminal Vi−2 and the output terminal Vo−2, and a resistor R2 is located between the input terminal Vi−3 and the output terminal Vo−2. Further, in the filter 15, a capacitor C3 is located between the input terminal Vi−3 and the output terminal Vo−3, a resistor R3 is located between the input terminal Vi−4 and the output terminal Vo−3, a capacitor C4 is located between the input terminal Vi−4 and the output terminal Vo−4, and a resistor R4 is located between the input terminal Vi−1 and the output terminal Vo−4.
Therefore, in the filter 15, the four capacitors C1 to C4 and the four resistors R1 to R4 are alternately arranged in that order so as to form a ring shape. The capacitors C1 to C4 are connected with the resistors R1 to R4 at eight connecting points, and the input terminals Vi−1 to Vi−4 and the output terminals Vo−1 to Vo−4 are alternately connected at the connecting points, respectively. The capacitors C1 to C4 have the same capacitance C, and the resistors R1 to R4 have the same resistance R.
The relation ω=1/(RC) is used by using an angular frequency ω. When four alternating current voltages having intensities and phases expressed by exp(jωt), exp(jωt), exp{j(ωt+π)} and exp{j(ωt+π)} are, respectively, applied to the input terminals Vi−1 to Vi−4, four alternating current voltages having intensities and phases expressed by exp(jωt), exp{j(ωt+π/2)}, exp{j(ωt+π)} and exp{j(ωt+3π/2)} are, respectively, outputted from the output terminals Vo−1 to Vo−4. In contrast, when four alternating current voltages having intensities and phases expressed by exp(jωt), exp{j(ωt+π/2)}, exp{j(ωt+π)} and exp{j(ωt+3π/2)} are, respectively, applied to the input terminals Vi−1 to Vi−4, four alternating current voltages having intensities and phases expressed by 20.5exp{j(ωt+π/4)}, 20.5exp{(ωt+3π/4)}, 20.5exp{j(ωt+5π/4)} and 20.5exp{j(ωt+7π/4)} are, respectively, outputted from the output terminals Vo−1 to Vo−4.
For example, Published Japanese Patent First Publication No. 2003-198329 discloses an active poly-phase filter in which two difference outputs, having phases different from each other by π/2 radians, are produced in response to high-frequency power applied to the filter.
Generally, the active poly-phase filter is composed of only passive elements, so that transmission loss is comparatively large in the filter. For example, when two filters 15 are serially connected with each other, the output power is reduced almost by 15 dB as compared with the input power. To prevent this power reduction, the output of each filter is amplified by transistors.
In the filter disclosed in this Publication, a differential amplifier is located on the front stage of the active poly-phase filter, and two emitters of two transistors composing the differential amplifier are connected with each other through a resistor. Because of the function of the resistor of the differential amplifier, the power range, in which the output power is linearly changed with the input power, can be enlarged.
However, because the resistor of the differential amplifier located on the input side of the active poly-phase filter acts as the feedback element of the emitters of the two transistors, the gain of the differential amplifier is undesirably reduced, and the input impedance of the filter is undesirably increased. Therefore, although the differential amplifier is located for the purpose of reducing the transmission loss caused in the filter, the transmission loss cannot be sufficiently reduced. Further, because the resistor of the differential amplifier determines the input impedance, the matching of the input impedance between the differential amplifier and the filter becomes very difficult. Moreover, when the input power applied to the differential amplifier exceeds the power range in which the output power of the differential amplifier varies linearly with the input power of the differential amplifier, an output signal transmitted from the differential amplifier to the filter is considerably distorted.